The present invention relates to the electronics arts. It is especially relates to flip-chip bonded group III-nitride light emitting diodes for lighting applications, and will be described with particular reference thereto. However, the invention also finds application in conjunction with other types of flip-chip bonded light emitting diodes, and in die-bonding of other optoelectronic devices such as vertical cavity surface emitting lasers (VCSELs).
In the flip-chip mounting configuration, a light emitting diode with a light-transmissive substrate and front-side electrodes is bonded “face down” to bonding bumps of a mount, that is, with the epitaxial layers proximate to the mount and the light-transmissive substrate distal from the mount. The flip-chip arrangement has a number of advantages, including improved thermal heat sinking due to the proximity of the front-side active layers to the heat sinking substrate, and elimination of electrode shadowing losses. In one such device, a group III-nitride light emitting diode that includes active group III-nitride layers on a light-transmissive sapphire or silicon carbide substrate is flip-chip bonded.
The p-type electrode of a flip-chip light emitting diode performs several tasks, including providing ohmic contacts to the active layers, efficiently reflecting light to contribute to light extraction, and providing thermal pathways for removing heat from the active layers during device operation. Moreover, the electrodes should be thermally stable at typical bonding temperatures, and should not degrade with use over time.
For group III-nitride light emitting diodes, nickel/aluminum (Ni/Al) and nickel/silver (Ni/Ag) reflecting electrodes for are known for contacting the p-type side of the diode. However, both these electrodes have problems. Ni/Al electrodes exhibit poor temperature stability, with substantial degradation of reflectivity at temperatures above 250° C. Ni/Al electrodes can show a reduction in reflectivity of about one-third, down to below 50% reflectance, after a 350° C. anneal, with corresponding decreases in light output of the light emitting diodes. Hence, Ni/Al electrodes are not thermally stable at temperatures typically used in soldering or otherwise flip-chip bonding the light emitting diode.
Ni/Ag electrodes tend to show better temperature stability. However, the silver of the Ni/Ag electrodes can migrate laterally with use over time. Migrating silver that reaches a sidewall of the device mesa produces an electrical shunting path that degrades device performance and can even produce catastrophic device shorting. Devices with Ni/Ag electrodes can exhibit catastrophic failure, and such catastrophic failure is typically experienced inside of 2000 operating hours.